SP212/3321&5521
Chapter 28 - Worksheet #4
Question 1: A very long, straight wire carries a current of i=2.50 A in the +x-direction in a region
with a uniform magnetic field B=3.50 T that lies in the xy-plane and is oriented 120◦ from the
x-axis as shown. What is the magnetic force acting on a 2.00 m long piece of this wire? Give your
answer as a vector.
B=3.50 T
y
120◦
i=2.50 A
x
+z is out of the page
Question 2: A circular wire loop with radius R=0.350 m lies in the xy plane. A current of 1.30 A
flows through the circuit, counter-clockwise when viewed from above. A magnetic field of magnitude
2.40 T passes through the circle, making a 35◦ angle with the z-axis. (a) What is the magnitude
of the torque acting on the wire loop? (b) The loop then rotates so that its magnetic moment is
parallel to the magnetic field (i.e. it rotates to its lowest potential energy position). What is the
change in the loop’s potential energy as it rotates?
B=2.4 T
◦
35
0.35 m
SP212/3321&5521
Chapter 28 - Worksheet #4
Question 3: A rectangular wire loop of dimensions 0.300 m by 0.200 m lies in the xy plane. A current
of 1.25 A moves clockwise around the loop when viewed from above. The magnetic field in this region
~
is uniform and is given by B=(0.275
î+0.450k̂) T. (a) What is the magnitude of the magnetic force
acting on the top section of the loop? (b) What is the magnitude of the torque acting on the wire
loop at this moment? (c) The loop then rotates so that its magnetic moment is parallel to the
magnetic field (i.e. it rotates to its lowest potential energy position). What is the change in the
loop’s potential energy as it rotates? [Note: part c is not easy. Think hard about how best to take
a dot product in both the initial and final orientations.]
i=1.25 A
y
0.200 m
x
0.300 m